Cellular mobile communications systems include stationary sites, referred to as base stations, and subscriber units, such as car telephones or hand-held telephones. A base station receives signals from and transmits signals to subscriber units which should be within a predetermined radius of the station. A number of base stations typically service a given geographical area, for example, a city.
The base stations and subscriber units transmit and receive signals in full duplex mode. The base stations receive signals and simultaneously transmit signals over a frequency range which extends from 800 MHz to 900 MHz (900 MHz to 1000 MHz using UK standards). The range includes two bands, an upper band and a lower band, which are separated in the frequency spectrum by about 45 MHz. The base stations transmit signals and the subscriber units receive them in one band, and the subscriber units transmit signals and the base stations receive them in the other band. This allows the base stations and the subscriber units to separate the received and transmitted signals.
Two types of antennas typically used in base stations are co-linear arrays and corner reflectors. Both types of antenna are to some degree capable of operating over a broad frequency range. They are thus capable of operating simultaneously over the two cellular communications bands. For example, co-linear arrays are typically optimized for one of the bands, e.g. the transmit band, and operate in a degraded manner over the other one.
What is needed is an antenna which operates without degradation over the entire cellular communications frequency range, that is, over both the upper and the lower bands. The antenna must also be economical to produce, rugged enough to withstand climatic changes, and essentially maintenance free. If such an antenna is light-weight, also, it can be easily mounted at the base station and/or used at the subscriber units to extend the range of the unit.